1. Field of the Invention
The present invention relates to a boundary microphone and a boundary microphone adapter capable of changing a sound collection axis of a microphone relative to a boundary plate.
2. Description of the Related Art
A boundary microphone (on-surface sound pickup microphone) is usually used in a conference room or a broadcast studio.
The boundary microphone is placed on a table when used in a conference room and placed mainly on the floor when used in a broadcast studio.
As disclosed, for example, in JP H8-65786 A and JP 2013-527995 W, the boundary microphone is configured with a boundary plate and a microphone held on the boundary plate.
In this configuration, the microphone is disposed close to the boundary plate, so that the microphone receives direct sound and reflected sound from the boundary plate. Since the direct sound and the reflected sound from the boundary plate have almost no time-gap (phase difference), a clear high acoustic signal can be obtained.
In the boundary microphone, for example, the sound collection axis of a unidirectional microphone is usually fixed parallel with the top face of the boundary plate.
Thus when a sound source is not on the sound collection axis of the boundary microphone, it is difficult to collect sound with good quality.
For example, if the boundary microphone is used in a conference room, the frequency response property of the boundary microphone placed on a table varies significantly between conditions when a speaker is sitting and when a speaker is standing.
Regarding such problem, a configuration may be employed in which the microphone unit is attached to the boundary plate with changeable elevation angle of the sound collection axis of a unidirectional microphone unit. In such a configuration, the sound collection axis of the microphone unit can suitably be adjusted toward a sound source, and deterioration in frequency response property caused by the deviation of the sound collection axis from the sound source can be prevented.
Meanwhile, when the sound collection axis of a microphone unit placed on the boundary plate is turned, the directional property of the microphone unit disadvantageously changes as explained below.
FIGS. 5A to 5C illustrate an external configuration of a unidirectional electric condenser microphone unit 1 used as a boundary microphone. FIGS. 5A to 5C are respectively a front view, a side view, and a rear view of the microphone unit 1.
As illustrated in the FIGS. 5A to 5C, the unidirectional electric condenser microphone unit 1 includes a cylindrical unit case 2 made of, for example, aluminum including an outer case, and as illustrated in FIG. 5A, a plurality of openings 3 serving as a front acoustic terminal of the microphone unit 1 provided in the front face of the unit case 2.
As well known, a diaphragm and a fixed pole facing the rear side of the diaphragm are disposed in the unit case 2, although not illustrated in the drawings. A circuit board 4 having an FET that functions as an impedance converter mounted thereon is attached to the rear side of the unit case 2 so as to seal the unit case 2 as illustrated in FIG. 5C.
The circuit board 4 is provided with a plurality of openings 5 which serve as a rear acoustic terminal of the microphone unit.
The microphone unit 1 is attached to the boundary plate 21 as illustrated in FIG. 6A. The microphone unit 1 is attached to the boundary plate 21 in a suitable configuration capable of changing the direction of sound collection axis D relative to the boundary plate 21 from the direction parallel with the top face of the boundary plate 21, or an elevation angle of 0 degree, to the vertical direction, or an elevation angle of 90 degrees.
FIG. 6B illustrates the microphone unit 1 in an attitude where the sound collection axis D is at an elevation angle of approximately 45 degrees. In this attitude, compared with an attitude where the sound collection axis D is at an elevation angle of 0 degree as illustrated in FIG. 6A, the rear acoustic terminal of the microphone unit 1 is positioned close to the boundary plate 21 and the gap between the rear acoustic terminal and the boundary plate 21 is narrow.
When the sound collection axis D is further turned toward the vertical direction, increasing the elevation angle, the gap between the rear acoustic terminal and the boundary plate 21 is further narrowed.
Along with the change in the elevation angle of the sound collection axis D of the microphone unit 1, signal waves traveling toward the rear acoustic terminal is further limited. The microphone unit thereby changes its characteristics from unidirectional characteristics to characteristics close to omnidirectional. When this change is significant, a problem such as howling arises.